School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA.
Nature. 2011 Jan 20;469(7330):389-92. doi: 10.1038/nature09718. Epub 2011 Jan 5.
The properties of polycrystalline materials are often dominated by the size of their grains and by the atomic structure of their grain boundaries. These effects should be especially pronounced in two-dimensional materials, where even a line defect can divide and disrupt a crystal. These issues take on practical significance in graphene, which is a hexagonal, two-dimensional crystal of carbon atoms. Single-atom-thick graphene sheets can now be produced by chemical vapour deposition on scales of up to metres, making their polycrystallinity almost unavoidable. Theoretically, graphene grain boundaries are predicted to have distinct electronic, magnetic, chemical and mechanical properties that strongly depend on their atomic arrangement. Yet because of the five-order-of-magnitude size difference between grains and the atoms at grain boundaries, few experiments have fully explored the graphene grain structure. Here we use a combination of old and new transmission electron microscopy techniques to bridge these length scales. Using atomic-resolution imaging, we determine the location and identity of every atom at a grain boundary and find that different grains stitch together predominantly through pentagon-heptagon pairs. Rather than individually imaging the several billion atoms in each grain, we use diffraction-filtered imaging to rapidly map the location, orientation and shape of several hundred grains and boundaries, where only a handful have been previously reported. The resulting images reveal an unexpectedly small and intricate patchwork of grains connected by tilt boundaries. By correlating grain imaging with scanning probe and transport measurements, we show that these grain boundaries severely weaken the mechanical strength of graphene membranes but do not as drastically alter their electrical properties. These techniques open a new window for studies on the structure, properties and control of grains and grain boundaries in graphene and other two-dimensional materials.
多晶材料的性能通常由其晶粒尺寸和晶界的原子结构决定。这些效应在二维材料中应该更为明显，因为即使是线缺陷也可以分割和破坏晶体。这些问题在石墨烯中尤为突出，石墨烯是一种由碳原子组成的六边形二维晶体。现在可以通过化学气相沉积在米级尺度上生产单层原子厚的石墨烯片，这使得它们的多晶度几乎不可避免。理论上，石墨烯晶界具有独特的电子、磁性、化学和力学性能，这些性能强烈依赖于其原子排列。然而，由于晶粒和晶界原子之间存在着五个数量级的尺寸差异，很少有实验能够充分探索石墨烯的晶粒结构。在这里，我们使用新旧透射电子显微镜技术的组合来弥合这些长度尺度。通过原子分辨率成像，我们确定了晶界处每个原子的位置和身份，并发现不同的晶粒主要通过五边形-七边形对缝合在一起。我们没有单独对每个晶粒中的数亿个原子进行成像，而是使用衍射滤波成像来快速映射数百个晶粒和晶界的位置、取向和形状，其中只有少数以前有报道过。得到的图像显示了一个出人意料的小而复杂的由倾斜晶界连接的晶粒拼贴。通过将晶粒成像与扫描探针和输运测量相关联，我们表明这些晶界严重削弱了石墨烯膜的机械强度，但并没有像以前那样极大地改变其电性能。这些技术为研究石墨烯和其他二维材料中的晶粒和晶界的结构、性质和控制开辟了一个新的窗口。
